The invention concerns a refractometer using the limited angle method.
Refractometers using the limiting angle method have been used for a long time to measure the refractive index of a medium, particularly a liquid medium: a prism of known index is placed in contact with the medium the index of which is to be measured and the interface of these two media is illuminated with the aim of determining the minimum angle of incidence at which reflection is total, i.e., the limiting angle.
To determine the limiting angle, the surface separating the two media can, for example, be illuminated from all directions of incidence simultaneously, and by, for example, moving a small cell in front of the reflected beam, the angle of incidence can be determined at which the amount of light received by the cell varies rapidly. A large cell receiving the whole of the beam reflected at once can also be used, and on the image received by this cell the position of the separator can be determined.
Also, instead of illuminating the surface separating the two media from all directions of incidence simultaneously, a small pencil can be selected corresponding to one direction of incidence, and the position of this pencil can be made to vary. This pencil can be selected by means of a movable slit positioned in the pupil of the objective. A fixed cell with large enough dimensions to be reached successively by each of the beams reflected determines the position of the slit at which the illuminance of the cell varies rapidly.
These procedures, however, have certain disadvantages.
In particular, since the lamp either must illuminate from all directions of incidence simultaneously or must illuminate a slit which is movable in a beam, it is necessary to choose a lamp which is powerful enough and therefore with a relatively limited life.
In addition, the two procedures already described assume that the light source is very precisely positioned and that there is a directionally homogeneous distribution of illuminance. As a result, if the lamp must be changed relatively often, it will be difficult to position the new lamp in exactly the same position as the first.
In addition, movement of the cell or the slit is expensive and entails inaccuracies.
In the procedure in which there is illumination from all directions of incidence simultaneously, considerable diffusion and hence stray light is also present.
The procedures known to date are consequently relatively expensive and inaccurate, and the invention attempts to solve these problems.